Alkylbenzenes (phenyl-alkanes) are prepared by the alkylation of benzenes. Alkylbenzenes have found many utilities, the most prominent of which is to make alkylbenzene sulfonates for use in laundry detergents and similar products. The performance of the alkylbenzene sulfonate in a detergent composition will be affected by the nature of the alkyl group, for instance, its length and configuration, especially branching.
At one time, many household laundry detergents were made of branched alkylbenzene sulfonates (BABS). The standard process used by the petrochemical industry for producing branched alkylbenzene consists of oligomerizing light olefins, particularly propylene, to branched olefins having 10 to 14 carbon atoms per molecule and then alkylating benzene with the branched olefins in the presence of a catalyst such as HF. However, BABS were slow to biodegrade, and linear alkylbenzene sulfonates (LABS) and certain modified alkylbenzene sulfonates, which are referred to herein as MABS, which biodegrade more rapidly than BABS, are now used in many detergent formulations.
LABS are manufactured from linear alkylbenzenes (LAB). The standard process used by the petrochemical industry for producing LAB consists of dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of a catalyst such as HF or a solid, acidic catalyst.
It is well known that aromatic by-products are formed during the catalytic dehydrogenation of paraffins. For instance, the article starting at page 86 of the Jan. 26, 1970, issue of “Chemical Engineering” states that the product of the dehydrogenation of linear paraffins includes aromatic compounds. Without limiting this invention in any way, these aromatic by-products are believed to include, for example, alkylated benzene, dialkylated benzene, naphthalenes, other polynuclear aromatics, diphenyl compounds, alkylated polynuclear hydrocarbons in the C10-C15 range, indanes, and tetralins. Typically, from about 0.2 to about 0.7 mass-percent, and generally to the extent of no more than 1 mass-percent, of the feed paraffinic compounds to a dehydrogenation zone form aromatic by-products. Although some commercially available dehydrogenation catalysts are more selective than others at minimizing the formation of aromatic by-products, it is believed that these by-products are formed at least to a small extent at suitable dehydrogenation conditions in the presence of most if not all commercially available dehydrogenation catalysts.
These aromatic by-products are believed to be deleterious in benzene alkylation processes, especially those using solid, acidic catalysts. First, some of the aromatic by-products may deposit on the surface of the catalyst and as mentioned above deactivate the catalyst. Second, some of the aromatic by-products are alkylated by mono-olefins to form heavy alkylate. Third, some of the aromatic by-products pass through the selective alkylation zone unreacted, are recovered with the overhead liquid stream of the paraffin column and are recycled to the dehydrogenation zone. These by-products can thus accumulate to unacceptable concentrations. In the prior art processes employing a solid alkylation catalyst, the concentration of aromatic by-products in the stripping effluent stream can typically accumulate to 4 to 10 mass-percent, which leads to rapid deactivation of solid alkylation catalyst. Where the alkylation catalyst is HF in the prior art processes, the concentration of aromatic by-products in the stripping effluent stream can typically accumulate to 3 to 6 mass-percent.
Processes for removing the aromatic by-products that are formed during the catalytic dehydrogenation of paraffins are also known. Aromatic by-products, however, are difficult to separate using conventional distillation techniques due to the similarity in boiling points the paraffins and olefins. Suitable aromatics removal zones may be selected from any processing methods that exhibit the primary requirement of selectivity for the aromatic by-products. Suitable aromatics removal zones include, for example, sorptive separation zones and liquid-liquid extraction zones. See U.S. Pat. Nos. 5,276,231 and 5,334,793, the contents of each are incorporated herein by reference. Where the aromatics removal zone is a sorptive separation zone, a fixed bed or a moving bed sorbent system may be used, but the fixed bed system is more common. The sorbent usually comprises a particulate material.
In a fixed bed system, the sorbent is typically installed in one or more vessels in a parallel flow arrangement, so that when the sorbent bed in one vessel is spent by the accumulation of the aromatic by-products thereon, the spent vessel is bypassed while continuing uninterrupted operation through another vessel. A purge stream comprising a purge component, such as C5 or C6 paraffin (e.g., normal pentane), is passed through the spent sorbent bed in the bypassed vessel in order to purge or displace or sweep unsorbed components of the stream containing the aromatic by-products from the void volume between particles of sorbent. The purge component is sometimes referred to herein as the sweep fluid. After purging, a regenerant or desorbent stream comprising a desorbent component such as benzene is passed through the sorbent bed in the bypassed vessel in order to desorb aromatic by-products from the sorbent. Following regeneration, the sorbent bed in the bypassed vessel is again available for use in sorbing aromatic by-products.
Thus, a sorptive separation zone for removing the aromatic by-products typically produces three effluents, which approximately correspond to each of the three steps in the cycle of sorption, purge, and desorption. The composition of each of the three effluents changes during the course of each step. The first effluent, the sorption effluent, contains unsorbed components (i.e., paraffins and olefins) of the stream from which the aromatic by-products are removed, and also typically contains the desorbent component. With its decreased amount of aromatic by-products relative to the stream that is passed to the sorptive separation zone, this effluent is used further along in the process to produce alkylbenzenes. For example, if the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the sorption effluent contains mono-olefins and paraffins and thus passes directly to the alkylation zone.
The second effluent, the purging effluent, contains the purge component, unsorbed components of the stream from which the aromatic by-products were sorbed, and often the desorbent component. The purging effluent is sometimes referred to herein as the sweep effluent. The third effluent is the desorption effluent, which contains the desorbent component, the aromatic by-products, and the purge component. In the typical prior art process, the purging and desorption effluents are separated in two distillation columns. The desorption effluent passes to one column, which produces an overhead stream containing the desorbent and purge components and a bottom stream containing the aromatic by-products which is rejected from the process. The overhead stream of the first column and the purging effluent pass to a second column, which separates the entering hydrocarbons into an overhead stream containing the purge component and a bottom stream containing the desorbent component and unsorbed components of the stream from which the aromatic by-products are removed. The overhead stream of the second column is used as the purge stream. The bottom stream of the second column is used in the process to produce alkylbenzenes. In the example described above where the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the bottom stream of the second column contains benzene, mono-olefins, and paraffins and flows directly to the alkylation zone. U.S. Pat. No. 6,762,334 discloses the use of a divided wall column to effect in one column the two distillations described above.
The aromatic by-products are usually burned for fuel value or exported as a low value feedstock to an aromatics unit for recovering benzene, toluene and xylene values. Hence, the stream has little value. Thus, not only do the aromatic by-products represent a loss of paraffin feedstock, but also, costs must be incurred to remove these aromatic by-products from the olefin-containing stream. Accordingly, dehydrogenation conditions have been used to minimize the production of the aromatic by-products. Accordingly, the olefin-containing feedstock to an alkylation reactor often contains about 90 mass-percent paraffins. Unfortunately, this has resulted in inefficiencies in the process for making alkylbenzenes. While paraffins are relatively inert to the alkylation reaction, they must be removed from the alkylbenzene reaction product to meet product specifications. Typically the paraffins are removed from the alkylbenzenes by distillation under substantial vacuum, e.g., from about 2 kPa(absolute) (hereinafter kPa(a)) (0.3 psi(absolute) (hereinafter psi(a))) to 10 kPa(a) (1.5 psi(a)) to avoid distillation temperatures that can degrade alkylbenzenes. And the sizable concentration of paraffins in the alkylation reaction product renders their removal by distillation a significant capital and operating cost in the overall process.